This invention relates to a time temperature indicating device for visually indicating the passage of a certain time temperature integral.
With a large number of products which may deteriorate or perish during storage there is need for a simple, inexpensive and easily attachable time indicating device. Such a device should clearly indicate the lapse of a particular permissible period of time for storage or use.
For applications regarding products whose keeping quality decreases with increasing temperature, it is desirable that this factor be taken into account by the indicating device.
A known time temperature indicating device comprises a capsule containing an evaporating substance such as glacial acetic acid. The capsule is placed in a small plastic bag through the wall of which acetic acid vapour can slowly diffuse. Positioned near the bag is a wick which contains an acid-base indicator. The device is isolated from the ambient atmosphere in that is packaged with a material which is impermeable to gases. The indicator is activated by breaking the capsule. Acetic acid vapour will then slowly diffuse through the wall of the bag and penetrate into the end of the wick placed near the bag, which will cause a distinct color change in the acidbase indicator. With the passage of time more acetic acid is supplied and an increasingly large part of the wick will be discolored, which is still speeded up with increasing temperature. A graduated scale placed along the length of the wick indicates to what extent the product guarded by the indicating device is still keepable.
A disadvantage of such an indicating device is that it is rather susceptible to leakage of developed gas. Should the capsule break or leakage be caused in some other way, then the indicator will discolor too fast. Should the outer packaging material be damaged, then acetic acid vapour will escape and the indicator will respond too slowly or not at all.